Optic system for light attenuation

ABSTRACT

An optic system including an integrated variable optical attenuating device for attenuating an input optical signal, the integrated optical variable attenuating device including a first electrically controllable attenuating element and a second electrically controllable attenuating element cascaded together, the first attenuating element having a first range of optical attenuation smaller than that of the second attenuating element but requiring less input power than the second attenuating element to switch between levels of attenuation within the first range of attenuation, and wherein the optic system further includes electrical circuitry for controlling the first and second attenuating elements in coordination with one another so as to achieve the desired level of total attenuation of the optical signal.

The present invention relates to an optic system for attenuating anoptical signal, and more particularly to an optic system including anintegrated variable optical attenuating device.

An example of an integrated variable optical attenuating device isdescribed in co-pending UK application no. GB0019971.5, in which a pindiode is defined about a ribbed waveguide defined in asilicon-on-insulator (SOI) chip. It has been observed that whilst such adevice has the advantages of being able to provide a relatively widerange of levels of attenuation, of being relatively polarisationdependent over that wide range of levels and of exhibiting low insertionloss in the off state, it does require a disproportionate level ofelectrical input power to achieve high levels of optical attenuation.

The present invention provides an optic system including an integratedvariable optical attenuating device for attenuating an input opticalsignal, the integrated optical variable attenuating device including afirst electrically controllable attenuating element and a secondelectrically controllable attenuating element cascaded together, thefirst attenuating element having a first range of optical attenuationsmaller than that of the second attenuating element but requiring lessinput power than the second attenuating element to switch between levelsof attenuation within the first range of attenuation, and wherein theoptic system further includes electrical circuitry for controlling thefirst and second attenuating elements in coordination with one anotherso as to achieve the desired level of total attenuation of the opticalsignal.

As mentioned above, the first and second attenuating elements arecascaded together such that they act in series on the input opticalsignal. In the embodiments described later, the two attenuating elementsare placed adjacent to each other, but the advantages of the presentinvention may also be achieved with other optical components interposedbetween the two attenuating elements.

As mentioned above, the first attenuating element has a first range ofoptical attenuation smaller than that of the second attenuating elementbut requires less input power than the second attenuating element toswitch between levels of attenuation within the first range ofattenuation. In the case, for example, where both the first and secondattenuating elements are designed to give a low level (e.g.substantially zero) of attenuation at zero electrical input power, theelectrical input power that would be consumed by the first attenuatingelement to achieve a greater level of attenuation within the first rangeof optical attenuations is less than would be consumed by the secondattenuating element to achieve the same level of attenuation.

In one embodiment, the first attenuating element is electricallyswitchable between a first, low level of attenuation and a second, highlevel of attenuation, and the high-power attenuating element iselectrically controllable over a range of attenuation levels between thefirst and second levels of attenuation, and the electrical circuitry isprovided for switching the first attenuating element between the firstand second levels of attenuation and for controlling the secondattenuating element over a range of attenuation levels between the firstand second levels of attenuation in coordination with each other inaccordance with the desired level of total attenuation.

In one embodiment, the first attenuating element is of the type whoseattenuation/electrical input power characteristic exhibits varyingpolarisation dependency over a range of attenuation levels (i.e. rangeof levels of electrical input power), and the first and second levels ofattenuation at which the attenuating element is actually operated areselected as those levels of attenuation at which the polarisationdependency is at a minimum.

Embodiments of the present invention are described hereunder withreference to the accompanying drawings, in which:

FIGS. 1( a) and (b) are views of a system according to a firstembodiment of the present invention;

FIG. 2 is a schematic view of a system according to a second embodimentof the present invention;

FIG. 3 is a graph showing typical input power/attenuationcharacteristics for the low power attenuating elements used in thesystems shown in FIGS. 1 and 2;

FIG. 4 is a graph showing typical input power/attenuationcharacteristics for the MZI-type attenuating element for the componentTM and TE polarisations of an input signal; and

FIG. 5 is a schematic view of a system according to a third embodimentof the present invention.

An optic system according to a first embodiment of the present inventionis shown schematically in FIG. 1( a). The system includes a Mach-ZehnderInterferometer (MZI)-type attenuating element and an in-line attenuatingelements (such as, for example, a pin diode attenuating element)monolithically integrated in a silicon-on-insulator (SOI) chip andcascaded together along an integrated waveguide 8, which may, forexample, be connected to optic fibres at the input and output ends ofthe chip. The structure of the each of the two attenuating elements isshown in detail in FIG. 1( b).

The MZI-type first attenuating element 4 includes an additionalwaveguide 20 defined in the SOI chip 2. It is designed such that aportion of the power of an optical signal propagated along waveguide 8is split into the additional waveguide 20 at the input end and theportions of the signal in each waveguide are recoupled at the outputend, with most of the power of the recoupled signal propagated furtheralong waveguide 8. An electrically controllable element 22 is providedfor adjusting the refractive index of a portion of the additionalwaveguide (the reversible injection of charge carriers into thewaveguide) and consequently adjusting the effective path length of theadditional waveguide 20. By controlling the effective path length of theadditional waveguide 20, the phase difference between the signals in thetwo waveguides at the point where the signals in each waveguide arerecoupled at the output end can be adjusted, and hence the attenuationof the signal can be adjusted according to the constructive anddestructive interference effects induced by the controlled phasedifference.

The in-line attenuating element 6 could be a pin diode attenuatingelement that includes n-doped and p-doped regions on either side of thewaveguide 8, such that charge carriers can be injected into thewaveguide upon application of an appropriate voltage across the n-dopedand p-doped regions. The injection of charge carriers into the waveguideincreases the absorption of the waveguide with respect to the opticalsignal and thus increases the attenuation of the optical signal. Thedegree of attenuation depends on the amount of charge carriers injectedinto the waveguide, which in turn depends on the voltage applied acrossthe n-doped and p-doped regions. The pin diode attenuating element 6 mayhave a structure as described in co-pending UK patent applications no.GB0019971.5 or GB0104384.3, whose contents are incorporated herein byreference. Alternatively, a different type of in-line attenuatingelement that operates by absorption effects could be used.

A processor 10 is used to control the attenuating elements incoordination with each other in accordance with a desired level of totalattenuation input to the processor. The manner in which the attenuatingelements are controlled in coordination is discussed below. The desiredlevel of total attenuation will often depend on the desired level forthe average power of the output signal. For example, it may be desiredthat the power of the output signal lies within a certain range matchingthe characteristics of the component to which it is subsequentlydirected, such as a photodiode for converting the optical signal into anelectrical signal.

The embodiment shown in FIG. 2 is an example of an optic system showinghow the attenuating elements may, if desired, be controlled withreference to the power of the output signal. The system is similar tothat shown in FIGS. 1( a) and (b), with like components being designatedby like reference numerals. It differs in that the order of the twoattenuating elements is reversed with the pin diode attenuating elementbeing placed at the input end, and in that the residual portion of therecoupled signal in the additional waveguide 20 is directed by means ofan integrated waveguide 16 to a photodiode 18, which may be positionedat the edge of the chip 2. The residual portion of the power of therecoupled signal has a known relationship to the power of the recoupledsignal in the waveguide 8, and the processor 10 controls the twoattenuating elements in accordance with the electric signal generated bythe photodiode so as to maintain the average power of the output signalin the waveguide 8 at the desired level.

MZI-type attenuating elements (and other types of attenuating elementsthat operate by interference effects) only provide a limited degree ofmaximum optical attenuation, but do so for a relatively low power input(where it is designed to be non-attenuating in the off state (i.e. noelectrical power input)). In contrast, because pin diode attenuatingelements operate by absorption effects, they can be used to attenuate anoptical signal to greater degrees than MZI-type attenuating elements,but are relatively power consuming in that the required electrical powerinput typically increases quadratically with optical attenuation, asshown in FIG. 3 by the solid curved line.

In some applications, it is important that the optic system attenuatesan optical signal with relatively small polarisation dependency.Although pin-diode attenuating elements do have the advantage ofexhibiting minimal polarisation dependency, MZI-type attenuatingelements 4 (and other attenuating elements that operate on interferenceeffects) can have an attenuation/input power characteristic that isrelatively polarisation dependent. The potential polarisation dependenteffects of the MZI-type attenuating element are minimised in thisembodiment of the present invention by the following method ofoperation.

It has been observed that the difference in attenuation between the TMand TE modes for the MZI-type attenuating element varies with the levelof input power to refractive index controlling element 22, and that atypical relationship between the level of attenuation for each of the TMand TE modes and the level of electrical input power is as shown in FIG.4, where there are levels of attenuation where the polarisationdependency, i.e. the difference between the levels of attenuation forthe TM and TE modes, is minimized. For example, at input power A, boththe TM and TE modes are subjected to a common high level of attenuation,I₁, and at input power B, both TM and TE modes are subjected to a commonlow level of attenuation I₂.

This characteristic is exploited in the method of operation of thisembodiment of the present invention. Where the desired level of totalattenuation lies between I₂ and I₁, the power input to the MZI-typeattenuating element is controlled to be B (this could be zero inputpower, if the MZI-type attenuating element is designed accordingly), andthe pin diode attenuating element is controlled to provide theadditional level of attenuation necessary to achieve the desired levelof total attenuation. Where the desired level of total attenuation isgreater than I₁, the power input to the MZI-type attenuating element iscontrolled to be A, and the pin diode attenuating element is controlledto provide the additional level of attenuation required to achieve thedesired total level of attenuation.

Where the desired level of total attenuation may be greater than 2×I₁,the optic system can be modified as shown in FIG. 5 to increase thepower efficiency of the system at these high levels of totalattenuation. As shown in FIG. 5, the SOI chip 2 includes an additionalMZI-type attenuating element 4 cascaded together with the first MZI-typeattenuating element 4 and the pin diode attenuating element 6. Where thedesired level of total attenuation is greater than 2×I₁, both the powerinputs to the two MZI-type attenuating elements are controlled to be A,and the pin diode attenuating element is controlled to provide theadditional level of attenuation required to achieve the desired totallevel of attenuation.

For example, each MZI-type attenuating element could be operated byswitching between substantially zero attenuation (at zero electricalinput power and a −10 dB level of attenuation at an increased level ofelectrical input power, and the pin diode attenuating element could beoperated to provide any additional level of attenuation up to a maximumlevel over −20 dB depending on the desired level of total attenuation.

As shown in FIG. 4, the power input required to achieve the level ofoptical attenuation, I₁, is greater for the pin diode attenuatingelement (whose characteristic is shown by the solid curved line) thanfor the MZI-type attenuating element, whose input power to achieve thelevel of optical attenuation, I₁, is shown as a solid dot. The opticsystems and their method of operation described above therefore resultin improved power efficiency whilst minimising any polarisationdependent effects of the MZI-type attenuating element.

In the embodiments described above, 1×1 and 1×2 MZI-type attenuatingelements are used as a low power switch for switching between twodiscrete levels of attenuation. However, other types of low-powerswitches may alternatively be used such as digital optical switches,array waveguide gratings and other types of Mach-Zehnder switches, suchas a 2×2 MZ switch, which may each be monolithically integrated with apin diode attenuating element on an SOI chip.

Although not shown in the embodiments described above, other opticalcomponents may be defined in the SOI chip. For example, a demultiplexeror multiplexer could be defined on the chip, with each of the pluralityof input or output waveguides being provided with one or more MZI-typeattenuating element and a pin diode attenuating element.

Furthermore, although the embodiments described above are silicondevices, the present invention is also applicable to integrated devicesbased on other optical materials such as III–V semiconductors andpolymers.

The applicant draws attention to the fact that the present invention mayinclude any feature or combination of features disclosed herein eitherimplicitly or explicitly or any generalisation thereof, withoutlimitation to the scope of any definitions set out above. In view of theforegoing description it will be evident to a person skilled in the artthat various modifications may be made within the scope of theinvention.

1. An optic system including an integrated variable optical attenuatingdevice for attenuating an input optical signal, the integrated opticalvariable attenuating device including a first electrically controllableattenuating element and a second electrically controllable attenuatingelement cascaded together, the first attenuating element having a firstrange of optical attenuation smaller than a second range of opticalattenuation of the second attenuating element but requiring less inputpower than the second attenuating element to switch between levels ofattenuation within the first range of attenuation, and wherein the opticsystem further includes electrical circuitry for controlling the firstand second attenuating elements in coordination with one another so asto achieve the desired level of total attenuation of the optical signal,wherein the first attenuating element is electrically switchable betweena first, low level of attenuation and a second, high level ofattenuation, and the second attenuating element is electricallycontrollable over a range of attenuation levels between the first andsecond levels of attenuation, and wherein the electrical circuitry isprovided for switching the first attenuating element between the firstand second levels of attenuation and for controlling the secondattenuating element over a range of attenuation levels between the firstand second levels of attenuation in coordination with each other inaccordance with the desired level of total attenuation, and wherein thefirst and second levels of attenuation are levels of attenuation atwhich the polarization dependency of the first attenuating element is ata minimum.
 2. An optic system according to claim 1 further including anelement for measuring an output power of the signal, and wherein theelectrical circuitry controls the first and second attenuating elementsin coordination with one another with reference to the measured outputpower so as to achieve the desired level of total attenuation of theoptical signal.
 3. An optic system including an integrated variableoptical attenuating device for attenuating an input optical signal, theintegrated optical variable attenuating device including a firstelectrically controllable attenuating element and a second electricallycontrollable attenuating element cascaded together, the firstattenuating element having a first range of optical attenuation smallerthan a second range of optical attenuation of the second attenuatingelement but requiring less input power than the second attenuatingelement to switch between levels of attenuation within the first rangeof attenuation, and wherein the optic system further includes electricalcircuitry for controlling the first and second attenuating elements incoordination with one another so as to achieve the desired level oftotal attenuation of the optical signal, wherein the second attenuatingelement operates by absorption effects.
 4. An optic system according toclaim 3, wherein the first attenuating element is electricallyswitchable between a first, low level of attenuation and a second, highlevel of attenuation, and the second attenuating element is electricallycontrollable over a range of attenuation levels between the first andsecond levels of attenuation, and wherein the electrical circuitry isprovided for switching the first attenuating element between the firstand second levels of attenuation and for controlling the secondattenuating element over a range of attenuation levels between the firstand second levels of attenuation in coordination with each other inaccordance with the desired level of total attenuation.
 5. An opticsystem according to claim 3 further including an element for measuringan output power of the signal, and wherein the electrical circuitrycontrols the first and second attenuating elements in coordination withone another with reference to the measured output power so as to achievethe desired level of total attenuation of the optical signal.
 6. Anoptic system including an integrated variable optical attenuating devicefor attenuating an input optical signal, the integrated optical variableattenuating device including a first electrically controllableattenuating element and a second electrically controllable attenuatingelement cascaded together, the first attenuating element having a firstrange of optical attenuation smaller than a second range of opticalattenuation of the second attenuating element but requiring less inputpower than the second attenuating element to switch between levels ofattenuation within the first range of attenuation, and wherein the opticsystem further includes electrical circuitry for controlling the firstand second attenuating elements in coordination with one another so asto achieve the desired level of total attenuation of the optical signal,wherein the integrated optical variable attenuating device is asilicon-on-insulator device with the first and second attenuatingelements defined in a layer of silicon.
 7. An optic system according toclaim 6, wherein the first attenuating element is electricallyswitchable between a first, low level of attenuation and a second, highlevel of attenuation, and the second attenuating element is electricallycontrollable over a range of attenuation levels between the first andsecond levels of attenuation, and wherein the electrical circuitry isprovided for switching the first attenuating element between the firstand second levels of attenuation and for controlling the secondattenuating element over a range of attenuation levels between the firstand second levels of attenuation in coordination with each other inaccordance with the desired level of total attenuation.
 8. An opticsystem according to claim 6 further including an element for measuringan output power of the signal, and wherein the electrical circuitrycontrols the first and second attenuating elements in coordination withone another with reference to the measured output power so as to achievethe desired level of total attenuation of the optical signal.
 9. Anoptic system including an integrated variable optical attenuating devicefor attenuating an input optical signal, the integrated optical variableattenuating device including a first electrically controllableattenuating element and a second electrically controllable attenuatingelement cascaded together, the first attenuating element having a firstrange of optical attenuation smaller than a second range of opticalattenuation of the second attenuating element but requiring less inputpower than the second attenuating element to switch between levels ofattenuation within the first range of attenuation, and wherein the opticsystem further includes electrical circuitry for controlling the firstand second attenuating elements in coordination with one another so asto achieve the desired level of total attenuation of the optical signal,wherein the first attenuating element attenuates the input opticalsignal by splitting the input optic signal into portions, inducing aphase difference between the portions, and then recombining theportions.
 10. An optic system according to claim 9, wherein the firstattenuating element is electrically switchable between a first, lowlevel of attenuation and a second, high level of attenuation, and thesecond attenuating element is electrically controllable over a range ofattenuation levels between the first and second levels of attenuation,and wherein the electrical circuitry is provided for switching the firstattenuating element between the first and second levels of attenuationand for controlling the second attenuating element over a range ofattenuation levels between the first and second levels of attenuation incoordination with each other in accordance with the desired level oftotal attenuation.
 11. An optic system according to claim 9 furtherincluding an element for measuring an output power of the signal, andwherein the electrical circuitry controls the first and secondattenuating elements in coordination with one another with reference tothe measured output power so as to achieve the desired level of totalattenuation of the optical signal.